Rivet fabrication

ABSTRACT

A method of forming an integral rivet in a sheet element including providing cooperating male and female dies and an interposed sheet element, moving the male die toward the female die to draw an outwardly directed hollow boss. Continuing movement of the male die toward the female die to thin an annular region in the lower portion of the hollow boss by compression which establishes metal flow substantially entirely in the direction of the upper portion of the hollow boss. Continuing relative closing movement of the male die with respect to the female die to establish an annular band of thinned material extending upwardly toward the upper portion of the hollow boss, whereby the upper portion of the hollow boss has a thickness substantially equal to the thickness of the remainder of the sheet element. The sheet element may be peripherally restrained around the area in which the boss is formed during boss formation. Apparatus for fabricating an integral rivet in sheet material including a male die having a convexly curved sheet contacting surface and a rearwardly disposed generally cylindrical surface. A female die having a bore defining wall and a peripheral material supporting surface disposed substantially perpendicular to the bore defining wall. An annular concave transition surface connecting the bore defining wall and the material supporting surface. The radius of curvature of the sheet contacting surface and the radius of curvature of the annular concave transition surface being such that movement of the male die toward the female die establishes relative closing movement between the dies which occurs initially at the outer portion of the annular concave surface of the female die and progresses inwardly along the surface as male die movement continues.

United States Patent [1 1 1 3,750,606 Schultz Aug. 7, 1973 RIVET FABRICATION die toward the female die to draw an outwardly di- [75] Inventor: Fred R. Schultz, New Kensington,

[73] Assignee: Aluminum Company of America, Pittsburgh, Pa.

[22] Filed: Mar. 18, 1970 [21] Appl. No.: 20,703

[52] 11.8. C1. 113/1 F, 113/121 C, 29/522 [51] Int. Cl 521d 41/26 [58] Field of Search 113/116 PF, 121 A,

113/121 C, 1 F, 116 CC; 29/522, 522 R [56] References Cited UNITED STATES PATENTS 3,387,481 6/1968 Harvey et a1. 72/379 3,545,249 12/1970 Brown 72/354 3,477,608 11/1969 Fraze 220/54 2,106,647 1/1938 Neck 72/256 1,413,284 4/1922 Maloney 72/256 3,479,733 11/1969 Brown 29/509 3,307,737 3/1967 Harvey 220/54 3,366,086 1/1968 Fraze 113/121 A 3,638,597 2/1972 Brown 114/116 FF Primary Examiner-Charles W. Lanham Assistant ExaminerM. J. Keenan Attorney-Arnold B. Silverman [57] ABSTRACT A method of forming an integral rivet in a sheet element including providing cooperating male and female dies and an interposed sheet element, moving the male stantially equal to the thickness of the remainder of the sheet element. The sheet element may be peripherally restrained around the area in which the boss is formed during boss formation.

Apparatus for fabricating an integral rivet in sheet material including a male die having a convexly curved sheet contacting surface and a rearwardly disposed generally cylindrical surface. A female die having a bore defining wall and a peripheral material supporting surface disposed substantially perpendicular to the bore defining wall. An annular concave transition surface connecting the bore defining wall and the material supporting surface. The radius of curvature of the sheet contacting surface and the radius of curvature of the annular concavetransition surface being such that movement of the male die toward the'female die establishes relative closing movement between the dies which occurs initially at the outer portion of the annular concave surface of the female die and progresses inwardly along the surface as male die movement continues.

9 Claims, 12 Drawing Figures PATENTEU 3.750.606

' sum 1 OF 4 PATENIHJ M18 3.750.606

sum 3 0F 4 FIG. 2 f

IN VE/V TOR FRED R. SCHULTZ A I rorney PATENT-mus H973 3,750,605

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it W \\\\\\\\\W m, w W) H w m W m M, g F F RIVET FABRICATION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method and apparatus for fabricating a hollow integral rivet in a sheet material and more specifically it relates to such fabrication wherein the flow of the sheet material and shape and thickness of the integral rivet are precisely controlled in order to provide improved rivet strength.

2. Description of the Prior Art It is frequently advantageous to provide forjoinder of a sheet element to a second element by means of a hollow rivet which has been integrally formed within the sheet element. It has been known to employ such a joint in container end walls which are provided with weakened removable sectors and lever pull tabs secured thereto in order to facilitate ease of end wall severance to permit access to the contents of the container.

As this type of rivet is frequently required to withstand substantial forces during the rivet forming stage, the joining operation and container opening, it is essential that the rivet have sufficient strength to function properly during these stages. In order to provide for satisfactory performance when the sheet material is metal, such as aluminum, rivet formation is frequently accomplished through a multistage operation. During the first stage the metal is stretched or drawn to form a hollow boss or bubble of substantially larger diameter than the final rivet. This stage produces a bubble of reduced wall thickness with respect to the remainder of the sheet. Subsequently, the bubble is reformed into a hollow rivet having a transverse end wall and a cylindrical peripheral wall.

When such a rivet is employed in an easy open end container an apertured pull tab is positioned over the hollow rivet and the rivet is staked to provide an enlarged head which secures the tab to the sheet metal element.

Qne of the major problems encountered in such rivet formation and joinder is that the rivet is frequently not provided with sufficiently uniform thickness and as a result rivet failure occurs. Such failure may occur through fracture of the material during the reforming stage when the bubble or hollow boss is converted into the rivet. Another critical stage when undesired fracture may occur is during the staking operation when the rivet is headed to establish the joint between the sheet element and pull tab. Finally, the substantial forces applied to the rivet head during operation of the pull tab to open the container may result in rivet fracture rather than removal of the severable part of the container end wall. All of these forms of failure make it imperative that a certain minimum material thickness be maintained in the rivet.

The above discussed problems are of increased significance with respect to certain forms of container ends having opening devices which must bear a very small price in order to remain economically competitive. This need has resulted in the strong desire to employ thinner gauge materials such as aluminum in a thickness of about 0.008 to 0.009 inch in easy open container walls. The need for a certain minimum rivet wall thickness in order to secure adequate strength has limited adoption of the more economical thinner gauge materials.

Another difficulty is encountered with container designs having an opening device adapted to remove substantially the entire container end wall as a unit such as is generally required where solid or semi-solid contents are involved. This in a preferred form employs a single weakened line around the entire periphery of the container end wall closely adjacent the seamer chuck wall of a can. Formation of a rivet in such close proximity to the score line and chuck wall imposes a restriction on the region of the panel from which metal may be taken during hollow boss and rivet formation. This limits the diameter of the hollow boss and reduces the amount of metal available and requires alternate ways of providing adequate rivet thickness for proper rivet performance.

One further difficulty encountered with conventional rivet formation of this type is that the surface of the metal element which will be exposed to the container interior is generally provided with a protective coating to prevent damaging contact between the container contents and the metal end wall. In conventional rivet forming the coating may be substantially impaired and subsequent repair coating required. This not only requires additional handling in processing of container end walls during fabrication, but also increases the costs of materials.

SUMMARY OF THE INVENTION The above described problems have been solved by the method and apparatus of this invention.

The method of this invention provides cooperating male and female dies and an interposed sheet element. The male die is moved relative to the female die in order to draw an outwardly directed hollow boss. Continued relative closing movement of the male die compressively establishes an annular thinned region in the lower portion of the hollow boss causing metal flow substantially completely toward the upper portion of the hollow boss. This provides for a substantial boss thickness where it is needed most. Further closing movement of the male die progressively establishes an annular thinned band within the lower portion of the hollow boss with the displaced metal moving upwardly toward the upper portion of the hollow boss to provide a wall thickness substantially equal to the remainder of the metal element. An annular region surrounding the area within which the hollow boss is to be formed is preferably clampingly restrained to retain sheet element flatness and limit the sector of the metal panel from which metal is taken during boss fonnation. The boss is subsequently reformed into a hollow rivet without material reduction in wall thickness. No meaningful coating damage is produced by this method.

The apparatus of this invention includes a female die having a bore defining wall and an annular substantially perpendicular material supporting or contacting wall with an interposed transition surface having a concave operating die surface. The male die has a convexly curved sheet contacting surface and a generally cylindrical rearwardly disposed surface. The radius of curvature of the sheet contacting surface of the male die, which may be simple or compound, and the radius of curvature of the concave annular surface of the female die are such that closing movement between the male die and female die initially establishes compression of the hollow boss at the outer portion of the annular concave surface of the female die with continued movement of the male die into the female die creating progressive thinning of the hollow boss. An annular thinned band is thereby provided within the base or lower portion of the hollow boss with substantially all of the metal flow during formation of this thinned band moving toward the upper portion of the hollow boss to establish increased thickness thereof. The hollow boss is then reformed into a hollow rivet without material thinning of the wall.

Reforming is effected in such fashion that the annular thinned region is moved into the panel surrounding the reformed rivet and is not in the rivet per se. This eliminates potential rivet weakening as a result of the thinned region.

It is an object of this invention to provide a method of forming an integral rivet in a sheet element by first forming a hollow boss, a major portion of which has substantially the same thickness as the remainder of the sheet element, and subsequently reforming the boss to provide a hollow rivet of substantially the same thickness as the remainder of the original sheet element.

It is another object of this invention to provide such a method wherein a strong rivet of substantial wall thickness may be created from thin gauge materials even where the rivet must be formed closely adjacent a score line or seamer chuck wall.

It is another object of this invention to provide a method of forming a hollow boss of substantial height and a rivet of large diameter without creating unduly thinned regions within the rivet.

It is a further object. of this invention to provide a method of forming such a hollow boss by first drawing a boss and'then annularly compressing a lower portion of the boss to establish local thinning and metal flow substantially completely in the direction of the remainder of the boss and sequentially continuously thinning upwardly toward the remainder of the boss.

It is another object of this invention to provide apparatus in the form of male and female die elements having specifically configurated curved forming surfaces which provide the above described progressive annular continuous boss thinning and substantially unidirectional metal flow to thicken the remainder of the hollow boss wall.

It is yet another object of this invention to provide such method and apparatus which are adapted for use with a broad range of metal sheet elements having various gauges, alloys and tempers, while avoiding any material impairment of protective coatings on the sheet elements.

These and other objects of this invention will be more fully understood and appreciated from the following description of the invention, on reference to the illustrations appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a partial sectional elevational view of a form of apparatus of this invention prior to initiation of forming.

FIG. 2 shows a partial sectional elevational view of a form of apparatus of this invention with the workpiece clamped but the remainder of the working stage not yet initiated.

FIG. 3 illustrates the drawing stage of hollow boss formation in process.

FIG. 4 shows the compressive thinning portion of the forming operation.

FIGS. 5 and 6 show the successive stages of hollow boss reforming to establish the hollow integral rivet.

FIG. 7 is a sectional detail of the female die.

FIG. 8 is a sectional elevation showing the hollow boss configuration.

FIG. 9 is a sectional detail of the male die.

FIGS. 10 through 12 illustrate the staking operation which establishes the final joint between the integral rivet and the member being attached.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now more specifically to the drawings with reference to FIGS. 1 through 4, the sequence of operations in forming the hollow boss and apparatus used therein is shown somewhat schematically. FIG. 1 shows the sheet material 2 interposed between male die 4 and female die 6. The female die 6 is secured within support member 8. The male die 4 is mounted for reciprocal movement with respect to the female die. An annular pressure pad 10 is disposed in encircling spaced relationship with respect to male die 4.

In the illustration shown in FIG. 1 the sheet material, which may be metal, such as aluminum, has been inserted between the dies 4, 6 with male die 4 and pressure pad 10 in retracted position. As is shown in FIG. 2, male die 4 has a curved sheet contacting surface 16 and a connecting rearwardly disposed generally cylindrical surface 18. The pressure pad 10 has a cylindrical bore defining wall 20 which is spaced radially outwardly of cylindrical surface 18. Pressure pad 10 also has an annular sheet restraining surface 22.

The female die 6 has an axial recess defined in part by bore defining wall 30, an annular sheet supporting wall 32 and a transitional throat surface 34. The throat surface 34 has an annular concave working surface 36 and reversely curved surfaces 38, 40 connecting the annular concave surface 36 with bore defining wall 30 and sheet supporting wall 32, respectively. Curved surface 40 has a relatively large radius, preferably about 0.040 to 0.075 inch, in order to permit free movement of metalinto the female die 6 during the drawing stage. Annular working surface 36 not only provides guidance for the metal moved into female die 6 during the drawing stage of boss formation, but also cooperates with male die 4 to control compressive reforming of the boss. 1

In FIG. 2 the substantially flat sheet like element 2 has been continuously clampingly engaged between sheet restraining surface 22 of pressure pad 10 and sheet supporting wall 32 of female die 6. This restraint which is preferably spaced radially outwardly from the region within which the boss will be formed is sufficient to substantially completely eliminate slippage during the drawing or initial phase of hollow boss formation. It also serves to maintain the sheet element 2 flatness to promote uniform drawing of the hollow boss. In FIG. 2 the leading portion of sheet contacting surface 16 of male die 4 has reached the sheet contacting position.

Referring now to FIG. 3, the sheet material 2 remains engaged between pressure pad 10 and female die 6. The male die has moved downwardly into the female die 6 and has through establishing tension in the sheet drawn or stretched a portion of the sheet element 2 into a hollow enlargement 42 which may have a somewhat reduced thickness with respect to the remainder of sheet 2. At this point the first stage of hollow boss formation has been completed.

Referring now to FIG. 4, it is seen that the sheet periphery continues to be restrained between pressure pad and female die 6. The male die 4 has moved downwardly farther and has established compressive engagement of the base or lower portion of the hollow boss between sheet contacting surface 16 and annular concave surface 36 of female die 6. Reference herein to the lower or base portion of the hollow boss shall refer to the portion closest to the remainder of the sheet element 2, and reference to the upper portion of the hollow boss shall refer to the portion more remote from the remainder of the sheet element 2. This engagement is such as to establish compressively induced metal flow to provide an annular thinned region 50 within the base portion of enlargement 42. The configurations of sheet contacting surface 16 and annular concave surface 36 are such that substantially all of the metal displaced from annular thinned region 50 flows toward the upper extremity 52 of enlargement 42 to increase the wall thickness of the hollow boss above region 50. Continued downward movement of male die 4 into female die 6 results in progressive thinning of the base portion of enlargement 42 sequentially in a direction toward upper extremity 52. The compressively established progressive fiow, therefore, produces an annular band of material in the base of enlargement 42 which is thinner than the remainder of enlargement 42. While the amount of thinning depends in part upon the amount of metal one wishes to move, the thinning must not be so great as to unduly weaken that portion of the boss. A residual in the thinned area of less than 0.006 inch is generally undesirable. The remaining portion of the enlargement 42 has a thickness substantially equal to that of sheet material 2. As the ultimate thickness of this enlargement or hollow boss with respect to the remainder of the sheet is dependent upon the amount of thinning during the drawing operation as well as the amount of metal flow established during compressive forming, it will be appreciated that the wall thickness of the hollow rivet may differ from that of the sheet material to a certain minor degree and such departures shall be included within the expression thickness substantially equal to"'the sheet thickness as used herein.

As a result of the progressive nature of this forming operation, the volume of metal moved during any increment of compressive forming is rather small. This material reduces the amount of tool force required in forming and with the substantially unidirectional flow of metal permits refined control of hollow boss thickness. Also, the avoidance of simultaneous thinning of the annular band and substitution of progressive thinning of finite sectors results in reduction of damage to the coating (not shown) applied to the surface of sheet element 2 which faces male die 4.

Upon completion of this compressive forming stage of hollow boss formation, the male die 4 is retracted. The pressure pad 10 and female die 6 are then subjected to relative separating movement to free the sheet material 2 which is now provided with a hollow boss of substantial wall thickness outwardly of the annular thinned band.

Referring now to FIGS. 7 through 9, detailed illustrations of the die elements 4, 6 and hollow boss 54 are shown. It is seen in FIG. 8 that the boss 54 has a relatively narrow annular thinned region 50 in the base portion thereof. Exclusive of this thinned portion 50, the thickness of hollow boss 54 is substantially equal to that of sheet material 2.

The female die 6, as shown in FIG. 7, is provided with annular concave surface 36 which has a radius R taken above a center C which is disposed on the longitudinal central axis of the female die 6 at a position spaced outwardly beyond the sheet supporting wall 32. Reversely curved surfaces 38, 40 have radii R, R which are substantially smaller than R. The diameter across annular concave surface 36 is larger than diameter D taken across bore defining wall 30. This configuration produces a female die 6 having the concave surface 36 recessed with respect to sheet supporting wall 32 yet outwardly disposed with respect to bore defining wall 30 and of greater diameter than bore defining wall 30.

A form of male die 4 is shown in detail in FIG. 9. While convex sheet contacting surface 16 may have a single radius, it is preferred to provide a compound radius. The compound radius contributes materially toward preventing excess thinning of the hollow boss at the outer extremity of the male die 4 during the drawing stage. In the form shown, sector S adjacent the central longitudinal axis has a radius R taken at a center C. Sector S which is disposed radially outwardly of sector S, adjacent the periphery of surface 16, has a radius R taken at a center C and sector S has a radius R taken above a center C. Radius R is larger than radius R which in turn is larger than radius R. It is noted that centers C and C of R and R both are positioned on the longitudinal axis of male die 4. The curvature of sheet contacting surface 16 in combination with the curvature of annular concave surface 36 provide for the annular progressive compressive thinning of the hollow rivet to produce metal flow substantially completely into the hollow boss.

In the form illustrated, the cylindrical surface is of male die 4 has a diameter D which is greater than the diameter D of the bore of female die 6. Depending upon the initial thickness of sheet material 2 and the desired hollow boss wall thickness, the die configurations may be varied slightly to provide minor variations. The increased thickness of the hollow boss wall will, however, be the result of metal flow from the base of the hollow boss upwardly into the other portions. This flow will be initiated compressively within an annular lower portion of the hollow boss and be completed through continuous progressive thinning toward the upper portion of the boss or bubble.

In order to verify the effectiveness of the system of this invention in terms of effective forming with thin gauge material and the elimination of the need for repair coating, several tests were made.

EXAMPLE I A number of aluminum can ends having a thickness of 0.010 inch were provided with hollow bosses having a diameter of 0.450 inch and a height of 0.125 to 0. l 35 inch. The annular thinned region near the base of the hollow boss had a thickness of 0.0075 to 0.008 inch, while the remainder of the boss had a residual of 0.0095 inch. When the hollow boss was reformed into a hollow rivet the annular thinned area was positioned 0.1 inch away from the base of the rivet. The hollow unstaked rivet had a height of 0.12 to 0.125 inch. The rivets had a head residual of 0.008 inch. These ends employed several different aluminum alloys. In all instances an acceptable strong rivet was produced.

EXAMPLE 2 A number of ends of thinner material were converted to easy open container ends. The ends were formed of metal having a thickness of 0.008 inch and made from aluminum alloy 5082-l-l19. The hollow boss diameter was 0.315 inch and the height was 0.080 to 0.085 inch. The thinned area had a residual thickness of about 0.006 inch. The rivet had a head residual of 0.004 to 0.0045 inch and the thinned area was spaced radially outwardly from the rivet base by about 0.060 inch. These rivets provided the desired rivet strength and performed in the desired fashion.

EXAMPLE 3 A series of aluminum can ends made of alloy S182Hl9 were converted to easy open ends by the system of this invention. The metal had a gauge of 0.0135. Hollow bosses having a height of about 0.095 to 0.110 inch were produced. These were reformed into hollow rivets having a height of 0.095 to 0.111 inch. The rivets were staked with a pull tab in place. The resulting rivet heads had an average diameter of 0.153 inch and a transverse end wall thickness averaging 0.01 inch with a minimum thickness of about 0.008. The resultant ends had rivets of substantial strength, exhibited no undue weakening and were successfully employed without fracture.

F168. 5 and 6 illustrate the tooling and method employed to reform hollow boss 54 into rivet 60. Male reforming tool 62 has a base portion 64 and anupstanding cylindrical portion 66 which terminates in a convex sheet contacting wall 68. Female reforming tool 70 has a cylindrical bore defining wall 72 and an outer bearing surface 74 with a curved transitional surface 76. In reforming hollow boss 54 relative movement is established between male tool 62 and female tool 70 with the hollow boss interposed. FIG. 5 shows the process after initiation of relative die movement, while FIG. 6 shows the process after the hollow boss 54 has been reformed to substantially conform to portions 66, 68 of male tool 62. During this stage, the female tool 70 pushes the metal downwardly over the boss 54. In this final position the sheet material 2 surrounding the hollow rivet 60 is engaged between annular surfaces 64, 74 of tools 62, 70, respectively. During rivet forming, annular thinned region 50 serves as a restriction which resists flow of metal into the portions of sheet material 2 adjacent the rivet. The annular thinned region 50 is positioned within the horizontal panel portion and does not form part of either the rivet side wall 78 or the transverse end wall 80. It is also noted that the thickness of the rivet wall is substantially equal to the thickness of the sheet material 2.

FIGS. 10 through 12 illustrate the method of securing a pull tab element 90 to the sheet material 2 by means of integral rivet 60. The sheet material 2 is supported on staking anvil 92 which has annular anvil surface 94 and projecting boss 96 which enters into the hollow of rivet 60. An annular pressure pad 98 cooperates with anvil surface 94 to clamp the tab against the sheet material during the rivet staking operation. As is shown in FIG. 11, after the pressure pad 98 has secured the tab inposition with the hollow rivet passing through tab aperture 100 and the tab 90 in surface to surface contact with sheet material 2, staking punch 102 which is concentric with and adapted to be moved within pressure pad 98 moves downwardly. Continued downward movement of the staking punch 102 causes it to engage and axially compress the rivet to assume the head configuration shown in FIG. 12. Rivet head 104 has an enlarged diameter which overlies and secured tab to sheet material 2. It is further noted that the reduction in rivet height during staking is sufficient to place the inner surface of the staked end wall 106 adjacent to boss 96 of staking anvil 92.

This invention provides a uniquely designed anvil 92 which facilitates retention of the maximum amount'of metal in the rivet during staking to eliminate undue thinning. As is shown in FIG. 10, the annular anvil surface 94 is provided with an upper surface level 108 and a lower surface level 1 l0 separated by vertical step 112. The vertical step 112 is disposed inwardly from the outer portion of thinned region 114 and is preferably spaced inwardly with respect to the radially outer portion of the thinned region 114. As is shown in FIG. 1 1, upper annular surface 108 cooperates with pressure pad 98 to compressively clamp sheet 2 and tab 90. This compressive restraint serves to restrict the amount of flow in the panel radially outwardly and away from the rivet area. Thus, undue thinning of the rivet during staking is avoided. As is shown in FIG. 12, some minor radially outwardly directed flow of metal does occur and undersurface 116 of sheet element 2 tends to assume a generally uniform thickness with no more than minor evidence of thinning after the staking operation has been completed.

One additional feature of staking anvil 92 which is of significance is the curved transition surface 118 between upper anvil surface 108 and the upper surface of boss 96. The curved transition surface is provided with a relatively large radius in order to avoid undue thinning of the rivet during staking and provide a gradual curved transition between the staked rivet andthe remainder of the panel. 1n the preferred form of this invention this radius is about 0.010 to 0.030 inch. This avoids sharp corners which result in stress concentrations and enhance the possibility of rivet fracture in this area during operation of the opening device. This complementary contour between the river and panel with respect to transition surface 118 is shown in FIG. 12.

It will therefore be appreciated that the method and apparatus of this invention provide for the conversion of a sheet metal element to provide an integral rivet. As a result of the specific combination of drawing and progressive compressive thinning resulting in unidirectional metal flow into the upper portion of the hollow boss, the hollow boss is provided with substantial thickness so as to resist fracture during reforming into a hollow rivet, during staking and ultimately during opening of an easy open end device. The position of the thinned annular band on the hollow boss and the reforming operation are such that the thinned region is placed outside of the rivet and therefore does not result in impaired rivet integrity. The reformed rivet may be staked conventionally. All of this is accomplished without damage to the interior coating on the container end, thereby eliminating the need for repair coating.

It will be further appreciated that this system permits economical manufacture of pull tab container walls as it is adapted for use with thin gauge materials, permits the fabrication of high rivet structures closely adjacent to container chuck walls and eliminates the need for cost adding repair coating. The method and apparatus are broadly applicable to various materials, alloys, tempers and gauges.

Whereas particular embodiments of the invention have been described above for purposes of illustration, it will be apparent to those skilled in the art that numerous variations of the details may be made without departing from the invention as defined in the appended claims.

We claim:

1. A method of forming an integral rivet in a sheet element comprising,

providing cooperating male and female dies,

positioning said sheet element between said dies,

moving said male die toward said female die to draw an outwardly directed hollow boss in said sheet element,

continuing said movement of said male die to effect compressive annular engagement of a lower sector of said hollow boss between said dies in order to compressively thin an annular portion of said lower sector of said hollow boss by establishing metal flow substantially entirely in the direction of the upper extremity of said hollow boss,

continuing said movement of said male die to extend said annular thinned portion upwardly in said hollow boss by sequentially compressing adjacent portions of said lower sector in order to establish a hollow boss having an annular compressively reduced thinned band of a thickness less than the remainder of said boss originating within said hollow boss lower sector and extending upwardly and merging into an upwardly disposed portion of said boss having a thickness substantially equal to the thickness of the remainder of said sheet element exclusive of said hollow boss, wherein said hollow boss diverges toward said upwardly disposed portion in the sector disposed between mating sheet contacting surfaces of said dies, and

subsequently reforming said hollow boss to establish a hollow rivet having a transverse end wall and a cylindrical peripheral wall.

2. The method of claim 1 including,

applying continuous annular clamping restraint to said sheet element around the area of said sheet element within which said hollow boss will be formed to limit the portion of the sheet element from which metal will be provided in drawing said hollow boss,

said female die having a sheet contacting face, an

axial recess and a connecting throat, and

said throat adjacent said sheet contacting face provided with a radius of about 0.040 to 0.075 inch.

3. The method of claim 1 including,

reforming said hollow boss into said rivet so as to position said annular compressively reduced band in said sheet element radially outwardly of the said peripheral wall of said rivet and said reforming is effected without appreciable thinning of the hollow boss wall.

4. The method of claim 3 including,

applying peripheral clamping restraint to said sheet element at a position spaced radially outwardly from the region within which said hollow boss will be formed.

5. Apparatus for forming an integral rivet in sheet material comprising,

a female die element having a bore defining wall and a peripheral material supporting surface disposed substantially perpendicular to said bore defining wall,

transition surface connecting said bore defining wall and said peripheral material supporting surface,

male die mounted for relative movement with respect to said female die having a convexly curved sheet contacting surface and a rearwardly disposed generally cylindrical surface adapted to have no substantial penetration into said female die when said male die and said female die element are in relative closed position,

said transition surface of said female die having an annular concave sheet contacting portion, a first adjacent convexly curved portion connecting said annular concave portion with said bore defining surface and a second adjacent convexly curved portion connecting said annular concave portion with said peripheral sheet supporting surface,

the radius of curvature of said sheet contacting surface of said male die and the radius of curvature of said annular concave portion of said female die being such that closing movement between said male die and said female die produces relative closing movement between said sheet contacting surface and said annular concave portion which occurs initially at the outer portion of said-annular concave surface of said female die and progresses inwardly along said annular concave surface as said relative closing movement of said dies continues and said male die sheet contacting surface diverges from said female die annular concave sheet contacting portion as they extend toward said female die bore defining wall when said dies are in relative closed position, whereby said sheet element will be progressively compressively thinned as said male die is subjected to sequential closing movement with respect to said female die with metal flow within said sheet element being substantially entirely in the general direction of said female die bore defining wall.

6. The apparatus of claim 5 wherein said clyindrical portion of said male die element has a diameter greater than the diameter of said bore of said female die.

7. The apparatus of claim 6 wherein annular sheet restraining means having a substantially flat sheet engaging surface is disposed in spaced surrounding relationship with respect to said male die, and

said restraining means is adapted for relative reciprocating movement with respect to said female die.

8. The apparatus of claim 7 wherein the center of said radius of curvature of said sheet contacting surface of said male die is disposed on the longitudinal central axis of said male die, and

the center of said radius of curvature of said annular concave surface of said female die is disposed along an extension of the longitudinal central axis of said female die outwardly beyond said sheet contacting face.

9. The apparatus of claim 7 wherein 0nd radius of curvature, and said radius of curvature of said annular concave surface of said female die is smaller than said first radius of curvature of said male die.

* l t l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3, 750, 606 Dated August 7 1973 Inventor(s) Fred R. Schultz It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the References, Change 114/116 FF" U. S. Patent No. 3,638,597 to l13/l16 FF".

Col. 2, line 7 After wall" insert --'positioned-.

Col. 3, line 12 Change "per se" to 1 er se C01. 8, line 7 7 Change "secured Col. 9, line 11 Change "We claim" to --I claim Signed and sealed this 20th day of Novemoer 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. RENE D. TEGTMEXER Atte sting Officer Acting Commissioner of Patents F ORM PO-IOSO (10-69) USCOMM-DC GOING-P69 a us GOVERNMENT PRINTING OFFICE I969 o-aee-au, 

1. A method of forming an integral rivet in a sheet element comprising, providing cooperating male and female dies, positioning said sheet element between said dies, moving said male die toward said female die to draw an outwardly directed hollow boss in said sheet element, continuing said movement of said male die to effect compressive annular engagement of a lower sector of said hollow boss between said dies in order to compressively thin an annular portion of said lower sector of said hollow boss by establishing metal flow substantially entirely in the direction of the upper extremity of said hollow boss, continuing said movement of said male die to extend said annular thinned portion upwardly in said hollow boss by sequentially compressing adjacent portions of said lower sector in order to establish a hollow boss having an annular compressively reduced thinned band of a thickness less than the remainder of said boss originating within said hollow boss lower sector and extending upwardly and merging into an upwardly disposed portion of said boss having a thickness substantially equal to the thickness of the remainder of said sheet element exclusive of said hollow boss, wherein said hollow boss diverges toward said upwardly disposed portion in the sector disposed between mating sheet contacting surfaces of said dies, and subsequently reforming said hollow boss to establish a hollow rivet having a transverse end wall and a cylindrical peripheral wall.
 2. The method of claim 1 including, applying continuous annular clamping restraint to said sheet element around the area of said sheet element within which said hollow boss will be formed to limit the portion of the sheet element from which metal will be provided in drawing said hollow boss, said female die having a sheet contacting face, an axial recess and a connecting throat, and said throat adjacent said sheet contacting face provided with a radius of about 0.040 to 0.075 inch.
 3. The method of claim 1 including, reforming said hollow boss into said rivet so as to position said annular compressively reduced band in said sheet element radially outwardly of the said peripheral wall of said rivet and said reforming is effected without appreciable thinning of the hollow boss wall.
 4. The method of claim 3 including, applying peripheral clamping restraint to said sheet element at a position spaced radially outwardly from the region within which said hollow boss will be formed.
 5. Apparatus for forming an integral rivet in sheet material comprising, a female die element having a bore defining wall and a peripheral material supporting surface disposed substantially perpendicular to said bore defining wall, a transition surface connecting said bore defining wall and said peripheral material supporting surface, a male die mounted for relative movement with respect to said female die having a convexly curved sheet contacting surface and a rearwardly disposed generally cylindrical surface adapted to have no substantial penetration into said female die when said male die and said female die element are in relative closed position, said transition surface of said female die having an annular concave sheet contacting portion, a first adjacent convexly curved portion connecting said annular concave portion with said bore defining surface and a second adjacent convexly curved portion connecting said annular concave portion with said peripheral sheet supporting surface, the radius of curvature of said sheet contacting surface of said male die and the radius of curvature of said annular concave portion of said female die being such that closing movement between said male die and said female die produces relative closing movement between said sheet contacting surface and said annular concave portion which occurs initially at the outer portion of said annular concave surface of said female die and progresses inwardly along said annular concave surface as said relative closing movement of said dies continues and said male die sheet contacting surface diverges from said female die annular concave sheet contacting portion as they extend toward said female die bore defining wall when said dies are in relative closed position, whereby said sheet element will be progressively compressively thinned as said male die is subjected to sequential closing movement with respect to said female die with metal flow within said sheet element being substantially entirely in the general direction of said female die bore defining wall.
 6. The apparatus of claim 5 wherein said clyindrical portion of said male die element has a diameter greater than the diameter of said bore of said female die.
 7. The apparatus of claim 6 wherein annular sheet restraining means having a substantially flat sheet engaging surface is disposed in spaced surrounding relationship with respect to said male die, and said restraining means is adapted for relative reciprocating movement with respect to said female die.
 8. The apparatus of claim 7 wherein the center of said radius of curvature of said sheet contacting surface of said male die is disposed on the longitudinal central axis of said male die, and the center of said radius of curvature of said annular concave surface of said female die is disposed along an extension of the longitudinal central axis of said female die outwardly beyond said sheet contacting face.
 9. The apparatus of claim 7 wherein said sheet contacting surface of said male die has a first radius of curvature adjacent the central longitudinal axis of said male die and a second radius of curvature radially outwardly thereof, said first radius of curvature is larger than said second radius of curvature, and said radius of curvature of said annular concave surface of said female die is smaller than said first radius of curvature of said male die. 